Some illuminating devices which use LED light sources include light guide members which guide lights from the LED light sources by totally reflecting the lights. The light guide member is a flat plate which has an emission surface, and emits lights to an outside from this emission surface. A rate of all light fluxes emitted from an illuminating device to all light fluxes of light sources is referred to as a light output ratio. Further, the illuminating device includes a heat transfer member which receives heat from LED light sources, and transfers heat to an outside of the device from a surface. In this case, preferably, heat is transferred not only from the surface of the heat transfer member but also from the surface of the light guide member. Then, the heat transfer member is provided in contact with the light guide member to transfer heat to the light guide member. However, it is known that the heat transfer member absorbs part of lights. As a result, the light output ratio of the illuminating device is reduced.